1. Field of the Invention
The present invention relates to methods for grinding side portions of hard, brittle material substrates, and more particularly, to a method for grinding edges and edge surfaces extending along the periphery of a substrate composed of a hard, brittle material, such as glass, quartz, ceramic, or sapphire (hereinafter, referred to as “hard, brittle material substrate”), and corners made of chamfered sections formed by cutting the aforementioned edges (the edges, the edge surfaces, and the corners of the edges will collectively be referred to as “side portion” in this specification).
In the present invention, the term “substrate” refers to a plate-like component on which a functional component is disposed for achieving a certain function, and includes a general so-called substrate, such as a glass substrate for a liquid-crystal display and a glass substrate for a hard disk, in addition to a glass cover with a liquid-crystal device for a portable phone and the like disposed on the rear face thereof so as to protect the device.
2. Description of the Prior Art
A glass substrate as an example of a hard, brittle material substrate is used as a flat-panel substrate in a liquid-crystal display for a liquid-crystal television, a personal computer, a portable information terminal, e.g. a portable phone, and a digital camera, or as a protective cover for protecting a liquid-crystal display. Moreover, since a glass substrate has low expansion and high impact resistance as compared with an aluminum substrate in the related art, the glass substrate is also used as a substrate for a hard disk. Thus, the industrial usage of glass substrates is increasing.
Such a glass substrate is cut into a predetermined shape, such as a rectangular shape if used as a flat-panel substrate or a donut-like shape if used as a hard-disk substrate, from a glass base material and is subsequently finished by grinding.
The grinding process for the glass substrate involves reducing the thickness thereof as much as possible or grinding a flat area thereof for improving the surface roughness. In addition, just after the glass substrate is cut, the edges (of the side portion) thereof tends to crack or chip easily. Moreover, if a crack or a micro-crack (small crack) formed in the cutting process remains in the side portion, the entire substrate may readily break from this cracked area when bending stress is applied to the substrate. Therefore, the grinding process is performed on the side portion so as to remove the edges by chamfering, and also to remove cracks and micro-cracks by polishing the edge surfaces and the chamfered sections to mirror finished surfaces.
Normal grinding methods that are currently performed for grinding the side portion of a glass substrate are roughly divided into a grinding method that uses a grindstone obtained by binding glass-grinding abrasive grains together with metal or resin as a binder and a grinding method that uses slurry containing abrasive grains.
As an example of the aforementioned grindstone-based grinding method, a method for grinding a hard-disk glass substrate by using a grindstone has been proposed. In this method, side portions of sheet-like glass substrates cut to predetermined dimensions are brought into contact with a rotating grindstone on a one-by-one basis. While monitoring the grinding amount, this grindstone is moved by NC control so as to chamfer the inner peripheral side and the outer peripheral side of each glass substrate and to grind the side portion thereof (Japanese Patent Laid-Open No. 2010-238310).
As an example of the aforementioned slurry-based grinding method, a method for grinding the inner periphery of an opening formed in the center of a hard-disk glass substrate has been proposed. When performing the grinding process with this method, a rotating brush is inserted into the central opening of a plurality of stacked glass substrates so as to bring the brush into contact with the inner peripheries of the openings. Then, the grinding process is performed by supplying slurry containing abrasive grains between the brush and the inner peripheries of the substrates at an appropriate timing (Japanese Patent Laid-Open No. H11-33886).
In the grinding method discussed in Japanese Patent Laid-Open No. 2010-238310, when using the rotating grindstone to grind the side surface and to chamfer the inner peripheral edge and the outer peripheral edge of each of the sheet-like glass substrates cut to predetermined dimensions, the grindstone is moved by NC control while the grinding amount is monitored. Thus, processing variations from product to product can be reduced, thereby allowing for a highly accurate process.
However, when glass, which is a hard, brittle material, is processed by using such a grindstone, seashell-shaped cut marks or chipped sections tend to form mainly at the end surfaces or at the corners if the workpiece is a plate. In addition, cracks or small cracks called micro-cracks (cut marks including such cracks and micro-cracks and occurrence thereof will collectively be referred to as “chipping” hereinafter) tend to form readily due to impacts caused during the cutting process.
These chipping tend to occur especially in sharp areas, such as the edges or the corners if the substrate is rectangular. If the side portion of the glass substrate is processed with an end mill in a front-end process prior to the grinding process, for example, grooves that correspond to positions where the end mill has passed and tool marks formed of sharp protrusions between the grooves are formed on the side portion, as shown in FIG. 13. If the substrate to be ground has such tool marks, the possibility of occurrence of the aforementioned chipping is further increased.
If the chipping is occurred, it is difficult to remove it completely by the grinding process. When bending stress is applied to the substrate, the glass substrate breaks readily starting from the chipped sections. Therefore, the strength of the glass substrate is significantly reduced.
In the grindstone-based process, the grindstone wears out and changes in shape as the processing amount increases. In addition, the grindstone clogs, resulting in lower grinding performance. Therefore, it is difficult to maintain constant processing quality, shape, and dimensions. If this is to be achieved, the processing amount needs to be monitored, and the grindstone needs to be replaced accordingly, making the management of the grindstone extremely complicated when performing the grinding process.
On the other hand, the slurry-based grinding process is performed by appropriately supplying slurry containing fine abrasive grains between a surface of the workpiece to be ground and the brush or a grinding pad that slides on the surface to be ground. Although the cutting performance with this method is lower than that in the grindstone-based grinding method, the occurrence of chipping can be significantly reduced even when grinding a glass substrate, which is a hard, brittle material substrate.
However, in this grinding method, when the slurry scattered throughout the working space dries, the fine abrasive grains in the slurry are scattered as dust and contaminate the working environment, which is a problem in that the dust may become a health hazard to workers.
In such a slurry-based grinding method, a relatively large amount of slurry is used since the slurry needs to be continuously supplied between the surface to be ground and the brush or the grinding pad. During the grinding process, the abrasive grains in the slurry break, changing the grain diameter, and heat generated due to the grinding causes the moisture to evaporate, thereby increasing the density of the abrasive grains. In addition, if foreign particles, such as shavings, produced from the grinding process are taken into the slurry, the foreign particles cannot be removed from the slurry. Thus, the quality of the slurry cannot be kept constant if the slurry is to be recycled, making it impossible to maintain the quality of the products.
Therefore, in the slurry-based grinding method, the slurry is normally disposed of after use, meaning that a large amount of abrasive grains is consumed as compared with the aforementioned grindstone-based grinding method.
Examples of abrasive grains generally used for grinding glass include fine diamond powder and fine cerium-oxide powder. Needless to say, diamond is an expensive material, and cerium oxide is also becoming an extremely expensive material because the worldwide demand therefor is increasing while cerium-oxide producing countries are putting stronger supply restrictions, such as mining restrictions, on cerium oxide. The use of disposable slurry containing such expensive materials as abrasive grains significantly increases the grinding costs.
In the brush-based grinding method discussed in Japanese Patent Laid-Open No. H11-33886, when grinding the inner peripheral surface of the stacked hard-disk glass substrates by using the rotating brush, as shown in FIG. 14, the grinding process is performed while supplying the aforementioned slurry between the brush and the surface to be ground. Therefore, this method is advantageous in that the occurrence of chipping can be prevented, as in the slurry-based grinding method described above.
Furthermore, since the grinding process is performed on the plurality of stacked glass substrates in the method discussed in Japanese Patent Laid-Open No. H11-33886, this method is advantageous in that the glass substrates can be simultaneously ground.
However, because the brush-based grinding method discussed in Japanese Patent Laid-Open No. H11-33886 is also a kind of slurry-based grinding method described above, there is a problem in that a large amount of expensive abrasive grains, such as fine diamond powder or fine cerium-oxide powder, is consumed.
Moreover, in the method discussed in Japanese Patent Laid-Open No. H11-33886, the brush, which is equipped with a shaft, used for the grinding is inserted into the central opening of the glass substrates in a state where only the upper end of the shaft is supported, as shown in FIG. 14. Therefore, even if a metal rod that is relatively resistant to deformation is used as the shaft of the brush, the lower end thereof wobbles during rotation such that the brush tips are not uniformly brought into contact with the surface to be ground. Therefore, when processing the stacked glass substrates, the degree of processing varies among the glass substrates in the height direction, which is a problem in that the quality varies from product to product.
In order to solve this problem, the brush would need to be moved vertically so as to reduce variations in the degree of processing in the height direction, or the grinding process would need to be performed multiple times while changing the stacking order of the glass substrates so as to make the degree of processing uniform. This results in lower workability due to a longer processing time.
Although glass substrates are described as an example of hard, brittle material substrates in the above description, the chipping may similarly occur when grinding substrates composed of a hard, brittle material other than glass, for example, quartz, ceramic, or sapphire, by using a grindstone. In addition, since expensive abrasive grains composed of diamond or cerium oxide are used, high grinding costs are required.
The present invention has been made to solve the problems in the related art described above, and an object thereof is to provide a method for grinding a side portion of a hard, brittle material substrate with good workability by preventing the occurrence of chipping, reducing the amount of abrasive grains consumed, preventing contamination of the working environment caused by abrasive grains, and allowing for a uniform grinding process for side portions of all substrates even when a plurality of stacked hard, brittle material substrates are simultaneously processed.